The present invention relates generally to an improved method for acquiring magnetic resonance images (MRI) of moving objects, and more particularly to, a method and apparatus to improve the efficiency of magnetic resonance coronary angiography (MRCA).
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Moving objects are particularly difficult to image, especially if an imaging plane is set in space with the object moving in and out of the imaging plane. Such imaging is especially difficult when a second periodic motion is added thereto. For example, imaging of objects in a subject which is breathing causes a periodic motion of internal structures, which is also further complicated by the beating motion of the heart if the structure is near the heart.
Acquisition of images during an end-expiratory breath-hold is commonly employed to minimize respiratory artifacts, while electrocardiography (ECG) gating can effectively freeze cardiac motion. Breath-held, ECG-gated two-dimensional (2D) CMRA can be accomplished using several imaging strategies, the most common being a 2D fast gradient-echo sequence segmented k-space acquisition (fgre). Two strategies for 2D CMRA are acquisition of the same slice over the entire cardiac cycle (traditional “CINE”) or acquisition of multiple slices with differing cardiac phases, typically acquired during mid-diastole. The prior art has successfully developed coronary artery imaging during the systolic phase, where a single image is acquired per acquisition. While such methods require that segments of the coronary artery be constrained within the plane of the prescribed slices, they do not make any implicit assumptions regarding the motion of the coronary arteries over the entire R-R interval. The visualization of the vessel-of-interest is therefore only ensured in a few frames.
Since there is substantial motion of the right coronary artery (RCA) and the left anterior descending (LAD) artery (in the order of 2 cm or more) during the cardiac cycle, the imaging efficiency (i.e., percentage of images containing a significant length of the vessel-of-interest) of these sequences is low. This implies that visualization of the vessel in its entirety generally requires several repeated breath-holds covering overlapping or contiguous slice locations, prolonging the scan times, which is generally not acceptable for patients with coronary artery disease.
The prior art proposed a method of tracking the motion of the coronary arteries prospectively across the cardiac cycle as a function of the delay from the cardiac trigger to improve the imaging efficiency. By adjusting the slice position as a function of cardiac phase, multiple images can be acquired in a single breath-hold, effectively tracking the vessel as a function of cardiac phase. This method reported an improved efficiency for the vessel tracking sequence compared to the multi-slice sequence. However, the prior art assumed a linear model for the motion of the vessel from its end-systolic to its end-diastolic position and back. While this linear model is often accurate in systole, during diastole, especially for the RCA, it is not. It has been found that the motion in diastole does not fit the linear model. As a consequence, the visualization efficiency in the diastolic phase, where the vessel moves the least, was less than optimal.
It would therefore be desirable to have a method and apparatus to improve the efficiency of acquiring MR images of a moving object by accurately, and automatically, tracking the moving object over a movement cycle. In particular, it is desirable to improve the efficiency of ECG-gated MRCA by accurate and automatic tracking of coronary vessel motion over the cardiac cycle.